With the need for smaller sizes, lower weights and higher functionality in portable electronic devices such as smart phones, digital cameras and handheld game consoles, active progress has been made recently in the development of high-performance batteries, and demand for secondary cells, which can be repeatedly used by charging, is growing rapidly.
Lithium ion secondary cells in particular, because of their high energy density and high voltage, and also because they lack a memory effect during charging and discharging, are the secondary cells currently undergoing the most vigorous advances in development.
Lithium ion secondary cells have a structure in which a container houses a positive electrode and a negative electrode capable of intercalating and deintercalating lithium and a separator interposed between the electrodes, and is filled with an electrolyte solution (in the case of lithium ion polymer secondary cells, a gel-like or completely solid electrolyte instead of a liquid electrolyte solution).
The positive electrode and negative electrode are generally produced by forming, on a current-collecting substrate such as copper foil or aluminum foil, a layer made of a composition that includes an active material capable of intercalating and deintercalating lithium, an electrically conductive material composed primarily of a carbon material, and a binder resin. The binder is a material that can be used to bond the active material with the conductive material, and also to bond these with the metal foil. Commercially available materials of this type include, for example, N-methylpyrrolidone (NMP)-soluble fluoropolymers such as polyvinylidene fluoride (PVdF), and aqueous dispersions of olefin polymers.
As part of recent efforts to tackle environmental problems, active progress is being made in the development of electrical vehicles. Lithium ion secondary cells are also expected to serve as the power source for such vehicles.
To this end, there exists a desire for lithium ion secondary cells which are endowed with even higher cycle characteristics, safety, capacity and output characteristics than up until now, and are also of lower cost.
It is known that batteries of high thermal stability and chemical stability can be produced by using sulfonylimide electrolytes as the nonaqueous electrolyte in lithium ion secondary cells. The reason is that, compared with the lithium hexafluorophosphate (LiPF6) commonly used as the electrolyte in lithium ion secondary cells, sulfonylimide electrolytes have a high thermal stability and hydrolysis stability.
However, a drawback with the use of sulfonylimide electrolytes is that the aluminum foil which is widely used as an electrode current-collecting substrate is corroded within the working voltage range of the cell, resulting in declines in the cycle characteristics and the capacity of the cell.
For this reason, sulfonylimide electrolytes are not commonly used.
To resolve this problem of corrosion, Patent Document 1 reports on the approach of adding a lithium salt such as LiPF6 or LiBF4 to a sulfonylimide electrolyte-containing nonaqueous electrolyte solution, and discloses that this can prevent the corrosion of an aluminum core serving as the current-collecting substrate of a positive electrode while suppressing a decline in the cell characteristics during high-temperature use and high-temperature storage.
The reason why this art can prevent the corrosion of an aluminum core is that, with cell charging and discharging, the LiPF6 or LiBF4 that is added forms a non-conductive film on the surface of the aluminum core, thus guarding against direct contact between the sulfonylimide electrode and the aluminum core.
However, when such an additive that forms a non-conductive film on a current-collecting substrate is used, the non-conductive film—which is a resistor—gradually grows with repeated charging and discharging, and so the resistance at the interface between the current-collecting substrate and the active material increases and the output characteristics of the cell decrease.
A drawback with the use of LiPF6 as the additive is that, owing to its low thermal stability and low chemical stability, improvements in the thermal and chemical stabilities of the cell are inadequate. When LiBF4 is used as the additive, the BF4− ions that form have a small diameter and interactions with lithium ions increase; hence, the degree of ion dissociation within the electrolyte solution decreases, resulting in a large cell internal resistance.
Patent Document 2 reports that by using as the current-collecting substrate an aluminum compact having an AlF3 film formed on the surface, reactions between the sulfonylimide electrolyte and the aluminum can be suppressed.
But because reactions between the aluminum and the sulfonylimide electrolyte cannot be adequately suppressed with an AlF3 film alone, here too, corrosion of the current-collecting substrate gradually proceeds and the cell capacity inevitably declines.
Patent Document 3 reports that by vapor depositing a lithium compound such as lithium fluoride or lithium carbonate onto a current-collecting substrate so as to provide a protective layer, reactions between a sulfonylimide electrolyte and the current-collecting substrate can be suppressed. However, drawbacks of this approach are the low productivity and high cost of the vapor deposition process.
Patent Document 4 reports that the corrosion of a current-collecting substrate by a sulfonylimide electrolyte can be suppressed by providing on the current-collecting substrate a protective layer made of, for example, a noble metal, an alloy, a conductive ceramic, a semiconductor, an organic semiconductor or a conductive polymer.
In this case, formation of a protective layer made of a noble metal or the like requires the use of a low-productivity, high-cost process such as vapor deposition, whereas a protective layer made of a conductive polymer can be formed by a high-productivity, low-cost coating process.
However, because conductive polymers themselves have a low redox resistance and low thermochemical stability, when formed into a protective layer, a sufficient cell capacity loss-suppressing effect tends not to be obtained.
In light of such advantages and drawbacks of conductive polymers, as disclosed in Patent Document 5, one approach under investigation is that of forming a conductive protective layer having dispersed therein a conductive carbon material of high thermal stability, high chemical stability and low cost.
However, it has been disclosed that, in conductive protective layers which use conductive carbon or graphite, because corrosion of the aluminum current-collecting substrate cannot be adequately suppressed, it is necessary to use also an additive such as LiBOB, LiFOB or LiPF6 which suppresses corrosion by forming a nonconductor film on top of the current-collecting substrate. In cases where such an additive that forms a nonconductor film on the current-collecting substrate is used, because the nonconductor film—which is a resistor—gradually grows with repeated charging and discharging, the resistance at the interface between the current-collecting substrate and the active material increases and the output characteristics of the cell decline.
Lithium iron phosphate (LiFePO4) is highly promising as a next-generation positive electrode active material because it has an excellent thermal stability, a large theoretical capacity of 170 mAh/g, and the lithium insertion-extraction reaction proceeds at a high potential of about 3.4 V (vs. Li/Li+).
However, it is known that lithium ion secondary cells in which LiFePO4 is used as the positive electrode active material tend to be unable to exhibit sufficient cycle characteristics.